This invention relates to a method and apparatus for reproducing information from a magnetic tape, especially a method for a variable speed reproduction by intermittently driving the magnetic tape.
In a rotating head helical scan type tape recorder (hereinafter VTR) of the prior art, a synchronizing signal is usually recorded on the magnetic tape during a recording mode, and the recorded synchronizing signal is reproduced during a reproducing mode for controlling a rotating phase of a capstan or a rotating cylinder by a servo circuit so that the rotating heads correctly trace the video tracks on which a video signal is recorded. Generally, for the synchronizing signal, control pulses (hereinafter CTL pulses), which are a series of pulses having a frequency one-half of that of a vertical synchronizing signal, are continuously recorded on one edge of the magnetic tape.
In a system using the CTL pulses, even if the magnetic tape is driven at variable speeds as occur with an intermittent drive for a noiseless slow motion picture reproduction, the position of the rotating heads with respect to the video tracks are absolutely determined at the time of detection of the CTL pulses. Therefore, in the system using the CTL pulses, it is easy to reproduce the video signal using an intermittent drive for the noiseless slow motion picture, in which the magnetic tape is stopped at one of the noiseless still states and is normally moved to the next noiseless still state, which is one frame period apart from the former one. By repeating the intermittent drive at a proper time-interval, the video signal is reproduced in noiseless slow motion. Accordingly, in the system using the CTL pulses, the variable speed reproduction by the intermittent drive is simply realized by executing a stop control which contains a reverse braking of a capstan motor after some time delay from the detection of one of the CTL pulses.
However, the above-mentioned reproduction cannot be used in other servo systems, for example, a system of the type in which pilot signals are recorded on the video tracks as synchronizing signals as disclosed in U.S. Pat. No. 4,056,832 and U.S. Pat. No. 4,297,733. The aforementioned patents only describe a system using pilot signals having three or four frequencies as a tracking signal for accurately reproducing the video signal at a normal tape speed.